A variety of iridium-based alloys have been proposed for use in spark plug electrodes to increase the erosion resistance of the firing surfaces of the electrodes. Iridium has a relatively high melting point and is more resistant to spark erosion than many of the metals widely used today. The iridium is typically used in the form of a pad or rivet that is laser welded or otherwise metallurgically bonded to the center and ground electrodes on either side of the spark gap. There are, however, known disadvantages to the use of iridium, including difficulty in bonding of the iridium to the electrodes and oxidative volatilization of the iridium at higher temperatures. The present invention addresses the latter of these two problems.
A known approach for reducing the oxidative loss of iridium is to utilize it in the form of an alloy combined with rhodium. U.S. Pat. No. 6,094,000 and published UK patent application GB 2,302,367 to Osamura et al. discloses such an alloy in which rhodium can be included in an amount ranging from 1-60 wt %. Group 3A and 4A elements such as yttria or zirconium oxide can also be added to help reduce consumption resistance. Notwithstanding Osamura et al.'s teaching of use of rhodium in amounts as low as 1%, it has been found that minimization of oxidative loss of the iridium at higher temperatures requires much higher amounts of rhodium. This is borne out in the test data presented by Osamura et al. and their patent notes that the amount of rhodium is preferably at least 3%.
U.S. Pat. No. 5,793,793 to Matsutani et al. reports a similar finding, wherein the amount of rhodium is kept within the range of 3-50 wt % and, most preferably, is at least 18%. In U.S. Pat. No. 5,998,913, Matsutani identifies some disadvantages of the inclusion of high percentages of rhodium and, in an effort to reduce the amount of rhodium in the alloy, proposes the addition of rhenium or ruthenium. According to this patent, by adding rhenium and/or ruthenium in amounts up to 17 wt %, the amount of rhodium needed to maintain good resistance to oxidative consumption can be lowered to as little as 0.1 wt %.